A pacemaker is a medical device, typically implanted within a patient, that provides electrical stimulation pulses to selected chambers of the heart, i.e., the atrium and/or the ventricle. Such stimulation pulses cause the muscle tissue of the heart (myocardial tissue) to depolarize and contract, thereby causing the heart to beat at a controlled rate.
Most pacemakers can be programmed to operate in a demand mode of operation, i.e., to generate and deliver stimulation pulses to the heart only when the heart fails to beat on its own. To this end, the pacemaker senses cardiac activity, i.e., heart beats, and if the heart beats do not occur at a prescribed rate, then stimulation pulses are generated and delivered to an appropriate heart chamber, either the atrium or the ventricle, in order to force the heart to beat.
When operating in a demand mode of operation, the pacemaker defines a period of time, referred to generally as the "escape interval" (which may further be referred to as either an "atrial escape interval" or a "ventricular escape interval," depending upon the mode of operation of the pacemaker) that is slightly longer than the period of time between normal heart beats. Upon sensing such a "natural" (non-stimulated or non-paced) heart beat within the allotted time period, the escape interval is reset, and a new escape interval is started. A stimulation (or pacing) pulse will be generated at the conclusion of this new escape interval unless a natural heart beat is again sensed during the escape interval. In this way, stimulation pulses are generated "on demand," i.e., only when needed, in order to maintain the heart rate at a rate that never drops below the rate set by the escape interval.
The heart rate is monitored by examining the electrical signals that are manifest concurrent with the depolarization or contraction of the myocardial tissue. The contraction of atrial muscle tissue is manifest by the generation of a P-wave. The contraction of ventricular muscle tissue is manifest by the generation of an R-wave (sometimes referred to as the "QRS complex"). The sequence of electrical signals that represent P-waves, followed by R-waves (or QRS complexes) can be sensed from inside of or directly on the heart by using sensing leads implanted inside or on the heart, e.g., pacemaker leads; or by using external electrodes attached to the skin of the patient.
All modern implantable pacemakers are programmable. That is, the basic escape interval (atrial and/or ventricular) of the pacemaker, as well as the sensitivity (threshold level) of the sensing circuits used in the pacemaker to sense P-waves and/or R-waves, as well as numerous other operating parameters of the pacemaker, may be programmably set at the time of implantation or thereafter to best suit the needs of a particular patient. Hence, the pacemaker can be programmed so as to yield a desired performance.
The operation of a pacemaker as described above presupposes that a stimulation pulse generated by the pacemaker effectuates capture. As used herein, the term "capture" refers to the ability of a given stimulation pulse generated by a pacemaker to cause depolarization of the myocardium, i.e., to cause the heart muscle to contract, or to cause the heart to "beat." A stimulation pulse that does not capture the heart is thus a stimulation pulse that may just as well have not been generated, for it has not caused the heart to beat. Such a non-capturing stimulation pulse not only represents wasted energy--energy drawn from the limited energy resources (battery) of the pacemaker--but worse still provides the pacemaker logic circuits with false information. That is, heretofore the logic circuits of the pacemaker presuppose that each stimulation pulse generated by the pacemaker captures the heart. If the stimulation pulse does not capture the heart, then the pacemaker logic circuits are controlling the operation of the pacemaker based on false information, and may thus control the pacemaker in an inappropriate manner. There is thus a critical need for a way of determining whether a given stimulation pulse has effectuated capture.
While there are many factors that influence whether a given stimulation pulse effectuates capture, a principal factor is the energy of the stimulation pulse. The energy of the stimulation pulse, in turn, is determined by the amplitude and width of the stimulation pulse generated by the pacemaker. Advantageously, in a programmable pacemaker, both the amplitude and pulse width of the stimulation pulse are parameters that may be programmably controlled or set to a desired value.
An implantable pacemaker derives its operating power, including the power to generate a stimulation pulse, from a battery. The power required to repeatedly generate a stimulation pulse dominates the total power consumed by a pacemaker. Hence, to the degree that the power associated with the stimulation pulse can be minimized, the life of the battery can be extended and/or the size and weight of the battery can be reduced. Unfortunately, however, if the power associated with a stimulation pulse is reduced too far, the stimulation pulse is not able to consistently effectuate capture, and the pacemaker is thus rendered ineffective at performing its intended function. There is thus a continual need in the pacemaker art for a system and/or method for adjusting the energy of a stimulation pulse to an appropriate level that provides sufficient energy to effectuate capture, but does not expend any significant energy beyond that required to effectuate capture.
Heretofore, the most common technique used to adjust the stimulation energy to an appropriate level has been manually, using the programmable features of the pacemaker. That is, at the time of implant, the cardiologist or other physician conducts some preliminary stimulation tests to determine how much energy a given stimulation pulse must have to effectuate capture at a given tissue location. If the preliminary tests indicate that the capture threshold is high (compared to the average patient) then the lead will be repositioned until a "good" threshold is found. Once it has been determined that the thresholds are good, the stimulation electrode is then left in place and the amplitude and/or width of the stimulation pulse is set to a level that is typically 2 to 3 times greater than the amplitude and/or width determined in the preliminary tests. The increase in energy above and beyond the energy needed to effectuate capture is considered as a "safety margin."
During the acute phase, e.g., over a period of days or weeks after implant, the stimulation pulse energy needed to effectuate capture usually changes. This stimulation pulse energy is hereafter referred to as the "capture-determining threshold." Hence, having a safety margin factored into the stimulation pulse energy allows the stimulation pulses generated by the pacemaker to continue to effectuate capture despite changes in the capture-determining threshold. Unfortunately, however, much of the energy associated with the safety margin represents wasted energy, and shortens the life of the pacemaker's battery. Furthermore, after the acute phase (when the lead is considered in the chronic phase), the capture determining threshold is typically much lower than that determined at implant. If left unchecked, the safety margin determined necessary at implant is extremely wasteful during the chronic phase. What is needed, therefore, is a means of regularly checking the capture-determining threshold and adjusting the stimulation pulse energy accordingly so that energy is not needlessly wasted in a safety margin that is excessively large.
A common technique used to determine if capture has been effectuated is to look for an evoked response (ER) following a stimulation pulse. The "evoked response" is the response of the heart that results from the application of a stimulation pulse to the heart. When capture occurs, the evoked response is an intracardiac P-wave or R-wave (which typically has a different morphology, or wave shape, than does a P-wave or R-wave resulting from natural cardiac contractions) that indicates contraction of the respective cardiac tissue in response to the applied stimulation pulse. For example, using such evoked response technique, if a stimulation pulse is applied to the ventricle (hereafter referred to as a V-Pulse), any response sensed by the ventricular sensing circuits of the pacemaker immediately following the application of the V-Pulse is assumed to be an evoked response that evidences capture of the ventricles. Similarly, if a stimulation pulse is applied to the atrium, which pulse is referred to hereafter as an A-Pulse, any response sensed by the atrial sensing circuits of the pacemaker immediately following the application of the A-Pulse is assumed to be an evoked response that evidences capture of the atria.
One problem with evoked response detection is that the signal sensed by the ventricular and/or atrial sensing circuits immediately following the application of a V-Pulse and/or A-Pulse may not be an evoked response. Rather, it may be noise, either electrical noise caused, for example, by electromagnetic interference (EMI), or myocardial noise caused by random myocardial or other muscle contractions (muscle "twitching"). Alternatively, that which is sensed by the ventricular and/or atrial sensing circuits may be a natural R-wave or P-wave that just happens to occur immediately following the application of the non-capturing V-Pulse or A-Pulse.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult to deal with because it is usually present in varying degrees, is lead polarization. Lead polarization is caused by electrochemical reactions that occur at the lead/tissue interface due to the application of the electrical stimulation pulse, A-Pulse or V-Pulse, across such interface. (The lead/tissue interface is that point where the electrode of the pacemaker lead contacts the cardiac tissue. Such point is normally inside the atrium or the ventricle, assuming endocardial stimulation leads are employed.) Unfortunately, because the evoked response is sensed through the same electrode through which the A-Pulse or V-Pulse is delivered, the resulting polarization signal also present at such electrode can corrupt the evoked response sensed by the sensing circuits of the pacemaker. To make matters worse, the lead polarization signal is not easily characterized. It is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy, and many other variables, most of which are continually changing over time.
In each case, the result is the same--a false positive detection of the evoked response. Such false positive detection thus leads to a false capture indication, which in turn can lead to missed heartbeats, a highly undesirable situation. What is needed, therefore, is a technique for clearly distinguishing a true evoked response from other signals that may occur at the same time as an evoked response, but are not an evoked response. What is needed, therefore, is a technique for eliminating, or at least minimizing, the adverse effect that lead polarization has on the ability of the pacemaker sensing circuits to sense the evoked response.
It is known in the art to generate stimulation pulses in pairs separated by a time less than the natural refractory period of the heart. (The natural refractory period of the heart is that time period following depolarization or contraction of the cardiac tissue during which the cardiac tissue is not capable of depolarizing again. Such natural refractory period, which may be thought of as a repolarization period, may vary from 100-200 msec or more.) The two-pulse approach uses the first stimulation pulse to effectuate capture wherein the signal measured immediately thereafter includes both the lead polarization and the evoked response. The second stimulation pulse does not effectuate capture (because the heart muscle tissue is not capable of contracting at that point in time) and the signal measured immediately thereafter is assumed to include only the lead polarization. The teaching of the prior art is that the signal measured after the second (non-capturing) pulse provides a measure of the lead polarization, which measure can then be electronically subtracted from the signal measured after the first (capturing) pulse to provide a true measure of the evoked response. See U.S. Pat. Nos. 4,674,508; 4,674,509; 4,708,142; 4,729,376; and 4,913,146, all issued to Robert DeCote, Jr.
There are two problems with the technique described in the DeCote, Jr. patents. First, it assumes that the "post-pulse lead recovery artifacts are essentially completely decayed within 50 msec. following the end of each pacing pulse," which is not universally true. Second, the invention by DeCote, Jr. requires an excessive amount of circuitry (e.g., an unsaturable sense amplifier, an A/D converter, an absolute value substractor, a digital integrator, a digital comparator, in addition to, threshold determination and control circuitry for carrying out the algorithm). Operation of this additional circuitry on a beat by beat basis simply draws too much current drain and can severely deplete the limited battery supply. Thus, there is a need for a system or technique whereby the evoked response signal can still be reliably sensed even in the presence of large polarization signals while requiring low power consumption. The present invention addresses the above and other needs.